16.04.11

Variable Voltage Control

Where fine control of both hoisting and lowering speed is required either booster control or a modified form of Ward-Leonard control is suitable and footbrakes are not essential. A magnetic brake provides against power failure or when returning the controller to off. Generally speaking for straightforward Ward-Leonard schemes the motor for the generator set can be either a.c. or d.c. and as the set runs continuously and in one direction only it is started in the conventional manner. Typical Ward-Leonard connections are shown in Fig. 19.12. If the supply is a.c. the exciter would be replaced by a static rectifier. An example of Ward-Leonard control for a combined cargo/mooring winch which enables the winch to comply with the St. Lawrence Seaway Regulations and so can be used when navigating the Seaway is shown in Fig. 19.13.


Under these Regulations vessels of 200 gross tons or less must have two lines of hawsers, one at the bow and the other at the stern quarter, each leading through a closed chock. Larger vessels must have at least four lines so arranged that they can be used on either side of the vessel. Two must lead from the bow and two from the stern quarters and not from the extreme bow or stern. For vessels between 200 and 300 gross tons the windlass forward and the capstan aft may be used for the two lines ahead but those leading aft must run from the main drum of power-driven winches and not from capstans or nigger heads. For all larger vessels all four lines must be power-operated and run from the main drum of power-driven winches and not from capstans or nigger heads.

Booster control systems are variations of the Ward-Leonard system except that the generator armature or booster as it is termed is connected in series with the winch motor. Performance can be said to equal that attained by conventional Ward- Leonard systems with the advantage of greater compactness which, for ship work, is a great asset. With Ward-Leonard the whole power for the winch motor is supplied by the generator of the motor-generator set so both machines of the latter are equal in output to the winch motor but with the booster system smaller motor-generator sets can be used as the booster is not called upon to handle full power. The fundamental circuits are shown in Fig. 19.14 and the complete system in Fig. 19.15. The booster voltage is reversible and can therefore add to the supply voltage or oppose it.

When adding to the line voltage the booster will be generating but when opposing it will be motoring the M.G. set and returning energy to the supply system. Since control is via the field system of the booster the change from step to step takes place smoothly and without jerk and there are no current peaks on the supply system. Main circuits are not broken while carrying current and the currents handled by the controller are small field currents with consequent elimination of excessive wear and burning of contacts. The power taken from the mains is proportional to the work done and not, as with conventional resistance control schemes, a constant demand while the winch is in operation, irrespective of the winch speed. Although this description is included under winches this system finds its most appropriate application for windlasses and capstans. An alternative to the booster system is the reducer which is connected as shown diagrammatically in Fig. 19.16.


Armatures A and B constitute a tandem set, both being on the same shaft, in which B supplies the motive power for A. It differs from the booster scheme in having the armature B in parallel to the capstan armature. When A is fully excited it generates full voltage in opposition to the line voltage and there is therefore no voltage on the winch motor. By gradually reducing the voltage of A the main motor voltage is correspondingly increased and it speeds up until eventually armature A idles round at zero voltage and the motor is at full running speed. In this system the reducer set has to be big enough to carry the full current required at slow speed but at top speed it is doing no work. The overall performance is such as to give the necessary falling characteristic.

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